The regulation of helper T cell clonal expansion, and differentiation influences the outcome of infections and the susceptibility to autoimmune diseases. Signal transduction pathways activated by cell surface receptors converge on transcriptional factors which determine effector responses by helper T cells. A long-term goal of this work is to characterize transcriptional mechanisms that regulate the potency of effector T cell responses in vivo as well as in vitro. When naive, uncommitted T helper (Th) precursors become Thi effectors that produce TNFs and 1FN-y they provide help essential to inflammation and the host defense against many pathogens. Because of this key role in host defense, the mechanisms by which signaling and transcriptional regulatory pathways control IFN-y production in vivo are of critical importance. The strength of an inflammatory effector response must be determined not only by the activation of specific cytokine genes (e.g., IFN-y) but also by efficiency of clonal expansion of antigen-specific T cells and rates of gene transcription after differentiation. Using a T cell-specific transgenic model to investigate the role of the NF-kB/Rel pathway in vivo, we discovered a preferential requirement for NF-kB in the type 1 (inflammatory) T cell-dependent response as compared to a type 2 (allergic) response. Preliminary studies provide evidence that clonal expansion in vivo is impaired, but also support the existence of a specific requirement for NF-KB in IFN-y gene activation. Using transgenic T cells and retrovector-mediated transduction, in Aim 1 we will distinguish the relative contributions of clonal expansion vs. effector cytokine gene activation, and identify mechanism(s) linking NF-kB to interferon gamma gene activation. Our hypothesis is that NF-kB has dual functions: one is preferentially to promote Thl clonal expansion by mediating protection against restimulation-induced death, and the other is to regulate IFN-y gene expression among surviving cells. To complement these studies, we will use knockout mice to test the hypothesis that one transactivating NF-kB subunit, RelB, plays a T cell-intrinsic role in Th1 development (Aim 2). In Aim 3 we will investigate the integration of T-bet and NF-kB/Rel pathways in regulating IFN-y gene expression. We propose to distinguish among models in which T-bet is downstream from or collaborates with the NF-kB/Rel pathway in regulating IFN-y transcription rates, versus one in which NF-KB is downstream from T-bet and in which access to NF-kB factors to regulatory sites in chromatin is enhanced by T-bet. Together, the proposed experiments will provide important insights into the transcriptional regulation of T cell-dependent inflammatory responses.